Half duplex and full duplex telephone interconnect operations on mobile radio systems are very common and fall into a number of general categories as follows.
Half duplex radios are known with push-to-talk (PTT) activated microphones. In these radios, the user accesses a telephone interconnect voice channel in a trunking or conventional radio system and dials a telephone number. When a corresponding land user answers, the mobile user will receive his voice via a speaker and use a hand-held microphone with PTT in order to initiate conversation. A handset (i.e. with microphone and earpiece) with a PTT button can also be used. Communication is in one direction at a time only and the mobile user has full control. When the PTT is not activated, the radio automatically switches to receive.
Full duplex radios are known with hands-free operation, in which the user accesses a telephone interconnect voice channel in a trunking or conventional radio system and dials a telephone number. When the land user answers, the mobile user receives his voice via a speaker and can use a hands-free vehicular speaker phone (VSP) microphone in order to talk. Because of the limited acoustic isolation between the receive and transmit audio paths in the vehicle cabin, the VSP microphone has to be partially or totally muted while the land user is voiced via the speaker, otherwise audio regeneration will appear. Sophisticated echo cancellers can be implemented using digital signal processors to provide hands-free full duplex operation. Unfortunately the circuitry involved is very expensive. For this reason, voice detection circuits with appropriate logic are used to decide "who is talking" and "who gets priority". Usually the land user will get priority to prevent an oscillatory condition (the land user's voice via the speaker can activate the voice detector of the mobiles transmit path). This arrangement actually yields a half duplex link, each of the users having to wait for the other party to finish before he starts talking. Common Motorola mobile cellular phones use this method. The land user has the priority and can break in with full volume at any time. The mobile user can break into the land user's transmissions with an attenuated level only ( 20-30 db down to normal) to attract the land user's attention. Sometime the low level audio is not sufficient to stop the land user's sentence and the mobile user has to pick up the handset to break in with full volume (real full duplex). When the user picks up the handset, a real full duplex link is established as in any regular home type telephone.
As well as the use of voice detection circuits for muting audio in full duplex radios and full duplex hands-free telephones for the purpose of isolation between receive and transmit paths, voice activity detectors are known in digital cellular radio systems (at both ends of a duplex communication channel) for switching into discontinuous mode transmission for battery saving and spectrum efficiency. Discontinuous mode transmission is a technique used in a full duplex digital link for preventing the transmission of carrier when there is no voice on the channel.
Voice detectors are also common in high frequency single-side-band radios. These are half duplex conventional radios where, in a radio-to-radio communication the voice operated switch is used as a PTT. Each party must wait for the other party to finish speaking before transmitting.
Among the drawbacks of a full duplex radio in a telephone interconnect system (when compared with a half duplex radio), are the following: a duplexer filter is essential to separate the receive and transmit signals and keep receiver performance at an acceptable level, this being expensive and bulky; receive and transmit frequencies must be well separated to make the required duplexer filtering achievable and this has spectrum allocation problems; at VHF, UHF and below, duplexers are currently even more bulky and expensive than at 800 MHz and above and need tuning for customer frequencies; at VHF and UHF trunking frequencies, duplexers are not feasible unless well separated frequencies are assigned for the site repeaters; full duplex radio has to handle continuous operation (telephone conversations often take up to 30 minutes or longer) at full power, so that immense heat dissipation capability is required (for example a 25 W output power radio at 800 MHz will have to dissipate up to 120 W continuously while the conversation is in progress); and a dual synthesizer loop or side step oscillator is essential provide to receive/transmit isolation.
By contrast to the above, a half duplex radio for a telephone interconnect system would have certain advantages including: wideband operation with antenna switching and no duplexer is required; transmit and receive frequency separation is not an issue; power amplifier RF power is well coupled to the antenna with very little loss; operation can be achieved in most bands with small size and low cost; there is lower heat dissipation (for the same output power, approximately 30% of the heat is dissipated compared to full duplex, due to the better efficiency and average of 50% transmit/receive duty cycle); and a single synthesizer loop can handle both the receive and transmit requirements.
GB patent application publication no. 2,021,902 describes a helmet-mounted half duplex radio-to-radio system having voice actuated circuitry that controls the transmit/receive mode of operation with no full duplex telephone connection. U.S. Pat. No. 4,524,461 also describes a helmet-mounted radio transceiver which has voice actuated circuitry that controls the transmit/receive mode of operation. The arrangement is generally half-duplex although a full duplex mode is described which requires two radio channels. In the latter arrangement, speaking into the microphone only disables the radio receiver.
It would be desirable to devise a communication system having the benefits of a half-duplex radio communication channel with improved management of a voice conversation between a land-line party and a mobile radio party.